Service de Physique de l'Etat Condensé

Marion Grzelka, Iurii Antoniuk, Éric Drockenmuller, Alexis Chennevière, Liliane Léger, and Frédéric Restagno

The role of the polymer volume fraction, ϕ, on steady-state slippage and interfacial friction is investigated for semi-dilute polystyrene solutions in diethyl phthalate in contact with two solid surfaces. Significant slippage is evidenced for all samples, with slip lengths b obeying a power law dependence. Navier’s interfacial friction coefficient, k, is deduced from the slip length measurements and from independent measurements of the solution viscosity η. The observed scaling of k versus ϕ clearly excludes a molecular mechanism of friction based on the existence of a depletion layer. Instead, we show that the data of η(ϕ) and k(ϕ) are understood when taking into account the dependence of the solvent friction on ϕ. Two models based on the friction of blobs or of monomers on the solid surface well describe our data. Both point out that Navier’s interfacial friction is a semi-local phenomenon.

https://doi.org/10.1021/acs.macromol.0c02804

Michal Swierczewski, Plinio Maroni, Alexis Chenneviere, Mohammad M. Dadras, Lay-Theng Lee, Thomas Bürgi

Nanoscale particles attract much attention due to their size-dependent optical, electrical and chemical properties. Of particular interest are ultrasmall metal nanoclusters which experience strong quantum confinement effect leading to profound changes in the atomic packing structure. The synthesis of these atomically precise metal clusters, typically with metal cores smaller than 2 nm in diameter, makes use of stabilizing functional molecules such as thiol ligands, hence deriving the common name – monolayer-protected clusters (MPCs). The next stage toward applications and at the same time a challenge in the field, is the manipulation and controlled organization of MPCs into two dimensional (2D) superlattices which would exhibit a collective response of the desired kind. Multiple examples of deposition techniques have been extensively studied, including droplet evaporation, spin-coating and chemical vapor deposition. However, a common drawback of all these methods is the failure to form large-scale structures of closely packed particles. Here, we study the formation and deposition of extended thin films of Au₃₈(SC₂H₄Ph)₂₄ nanoclusters onto solid supports by the Langmuir-Blodgett (LB) method. A combination of techniques, atomic force microscopy (AFM), high magnification transmission electron microscopy (TEM), X-ray reflectivity (XRR), and grazing incidence wide-angle X-ray scattering (GIWAXS) is applied to reveal the morphology and the degree of vertical and in-plane ordering of the transferred films. We find that while a degree of order is initially obtained between the clusters, temporal annealing of the compressed films successfully removes mesoscopic defects between islands of nanoclusters but that it does so at the cost of reducing the local order within the domains. To our knowledge, this is the first reported example of the deposition on an extended scale (several cm²) of ordered gold nanoclusters in the small size regime of 1 – 2 nm.

https://doi.org/10.1002/smll.202005954

Alessio Zaccone and Laurence Noirez, J. Phys. Chem. Lett.12 (2021) 1, 650–657.

Liquids confined to sub-millimeter scales have remained poorly understood. One of the most striking effects is the large elasticity revealed using good wetting conditions, which grows upon further decreasing the confinement length, L. These systems display a low-frequency shear modulus in the order of 1–10³ Pa, contrary to our everyday experience of liquids as bodies with a zero low-frequency shear modulus.

While early experimental evidence of this effect was met with skepticism and abandoned, further experimental results and, most recently, a new atomistic theoretical framework have confirmed that liquids indeed possess a finite low-frequency shear modulus G′, which scales with the inverse cubic power of confinement length L. We show that this law is universal and valid for a wide range of materials (liquid water, glycerol, ionic liquids, non-entangled polymer liquids, isotropic liquids crystals). Open questions and potential applications in microfluidics mechanochemistry, energy, and other fields are highlighted.

https://doi.org/10.1021/acs.jpclett.0c02953.

Andrew M. Jimenez, Dan Zhao, Kyle Misquitta, Jacques Jestin and Sanat K. Kumar

Understanding the structure and dynamics of the bound polymer layer (BL) that forms on favorably interacting nanoparticles (NPs) is critical to revealing the mechanisms responsible for material property enhancements in polymer nanocomposites (PNCs). Here we use small angle neutron scattering to probe the temporal persistence of this BL in the canonical case of poly(2-vinylpyridine) (P2VP) mixed with silica NPs at two representative temperatures. We have observed almost no long-term reorganization at 150 °C (∼T_g,P2VP + 50 °C), but a notable reduction in the BL thickness at 175 °C. We believe that this apparently strong temperature dependence arises from the polyvalency of the binding of a single P2VP chain to a NP. Thus, while the adsorption–desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a process that occurs over a seemingly narrow temperature range.

https://doi.org/10.1021/acsmacrolett.8b00877

( English version)